Welding heat control



2 Sheets-Sheet 1 IN vew TOR AT79ORNEY D. P. FAULK WELDING HEAT CONTROL March 16, 1954 Filed Dec. 29, 1949 DONALD P FA ULK BY Patented Mar. 16, 1954 7, 2,672,543 WELDING HEAT CONTROL Donald. P. Faulk, Lexington, Mass., assignor to Raytheon- Manufacturing Company, Newton, Mass., a corporation of Delaware Application December 29, 1949, Serial No. 135,589

7 Claims.

This invention relates to heat controltiming and more particularly to heat control timing in resistance welding.

In seam welding where rollers" re used as the welding electrodes, it has heretofore been customary to use an on and oil timing technique. That is, during the ,ffon time, inter mittent current pulses of constant amplitude were passed through the .weldmaterial at the point of'weld. During the .fofi time, no current was permitted to flow through the weld material. Such off time periods were found, necessary to prevent the pressure of the rollers from pushing through the molten weld material. The general practice has been to maintain a constant flow of coolant fluid at the point for vweld to confine the -welding heat to narrow limit of area. During the off time periods, "this coolant rapidly cooled and solidified the molten. weld material to give it suflicient body to withstand the pressure of the rollers in preparation-tor the succeeding on time welding period. The result of such a timing procedure was to produce a substantially rectangular graphical picturefof heat versus time at the point of weld. The inceptionof the on time caused a nearly vertical r ise in temperature of the weld material to a predetermined constant equilibrium welding temperature at which the weld material reached a I molten state. The travel of the electrode rollers along the weld seam at the equilibrium state was'fsuch that the temperature at the weld .remained'substantially constant throughout the fon time period. At the end of the on time period; the weld material was caused to drop nearly vertically in temperature due to the coolant. fluid, thus solidifying the molten weld material to provide a solid base for the rollers to ride over preliminary to another I onf time period. I

There are many disadvantages to such a heat control plan for scam welding. Because of the extremely rapid rise in temperature atthe weld point, a poor weld may result. ;For example, if a small piece of gas emitting scale exists between the weld members, as often occurs when hot rolled steel is usedias the weld material, this rapid temperature rise results in-high gaseous pressures for lack of time for the gas-to escape. The rapid pressure rise in, many instances is suiiicient to cause localized explosions creating holes and generalporosity atthe weld.-, -Also, the rapid rate of cooling causes brittlenessat the weld when carbon steels are used, thereby male-1 ing further. iabrication oi the ,material impossible. For example, in the manufacture of steel drums from seam welded sheet steel, a brittle weld tends to tear when the annular reinforcing grooves are formed. Another difficulty is that, when some stainless steels are used as the weld materials, such rapid cooling from the molten state causes destruction of the stainless properties.

The present invention overcomes these defects by providing a novel heat-timing technique. There is no on and ofi timing used. Rather, there is effected a gradual rise in weld temperature to a predetermined peak after which the heat cycle is repeated. The heating cycle may be graphically approximated as a saw-tooth wave in which the slope, time interval between cycles, and maximum and minimum temperatures may be varied to accommodate any particular ma terial. .Such a relatively gradual increase in heat intensity in the weld prevents sudden high gas pressure spots. The minimum temperature in the cycle is just low enough to prevent breakthrough of the welding electrodes, and yet not so low as to have caused a brittle weld.

Such a heating cycle is accomplished by progressively increasing the intensity of the current pulses at the weld to a selected peak after which the cycle is repeated. The present embodiment for accomplishing this consists generally of inverse parallel connected ignitrons arranged in an alternating current power line and fired by thyratrons at progressively advancing points in the alternating potential cycle of the power source. A base biasing potential, 2. sawtooth potential generator, and a step potential generator for progressively increasing the base potential of the saw-tooth potential wave are connected across the grid and cathode of the thyratrons. The saw-tooth and step potential generators are synchronized with the alternating potential source so that, as the base potential rises with each step of the step generator, the thyratron are made to fire at progressively advancing points in the cycles of the alternating potential source, and thus cause the ignitrons to conduct at corresponding points. One ighitron conducts in the positive half-cycle while the other conducts in the negative half-cycle of the alternating potential source. The firing point in the negative and positive portion of each cycle is maintained the same to prevent current unbalance and consequent saturation losses in the welding transformers fed by the ignitrons.

A timing device is arranged to return the step generator to a-base potential at desired intervals determined by its adjustment and thereby provide a time cyclic return to a base firing position. Controls are also provided for adjusting the degree of base biasing potential and the increments between steps of the step potential generator thereby permitting selection of the position of the base firing point and the rate of advance of the succeeding firing points in the cycles of the alternating, potential source.

The foregoing and other advantages, objects, and features of the invention will be better understood from the following description taken in connection with the accompanying drawings inwhich: 3

Fig. 1 is a schematic drawing of a preferred embodiment of the invention; 7

Fig. 2 is a diagram illustrating the operation of a saw-tooth potential wave generator used in Fig. 1;

Fig. 3 is a diagram illustrating the operation of a step potential timer used in Fig. 1; and

Fig. 4 is a series of graphs illustratin the overall operation of theembodiment in Fig. 1.

"Referring to the drawings in more detail, a welding transformer T having a secondary winding '11 for "connection to "a resistance weldingl'oad has a winding l2 adapted to be-cohne'ctedto an alternating current power source (not shown) across lines l4 and f5. Any

' variable resistance 52.;

suitable alternating current power source may bGhSlfd. A pair "of inverse parallel connected ignitrcnsl6 and i-Tare connected "in series with the"-prirna1'y-winding 12' across the alternating current power source lines 14 and I5. The thy'ratron 'tubes18' and 19 "are connected for firing the ignitrons leand ll, respectively. The

anode 28-of 'thyratron T8. "is connected through resistance 21 at the point2'2 to the anode 23 or the ignitron it. The anode 24 of the thyra'tron I9" isconnected through resistance 25 to the point 2 8 at the anode 2lof the ignitron ll.

*Cathodes 28 and 290i 'thyratrons i3 and Is, respectivelyrare connected to igniting or controlling" electrodes '38 and3l of 'ignitrons IBa'nd ll, respectively-"The f'gri'd 326i 'thyratron I8 is connected through line 33' in. series with a condenser-34, a condenser 35; and a grid biasing constant potential source 36 to the cathode 28 of thyratron t-Bbyline 3I andigniting electrode 30. A grid-3e of thyratron "l9 is'corinectedthrough a secondarywfl of nullifying'transformer '48 to oint 41 in line 33: A primary c2 of nullifying transformer i'sconnected across the cathodes 43 and- 44-of ignitrons "I 6 and H, respectively, to preventthe potential-from lines l4 and IBfrom appearing atth'e grid "38 thereby permitting "controt-of both thyratrons t8 'and I9 from the single point 4|.

A suitable constant'potent'ial source 36 comprises'a transformer 45 having a primary '46 con nected-acrossthe power source lines "M and I5; and a secondary '48" inseri'es with a half-Wave rectifying device 41 positioned to provide across a smoothing condenser 483a potential polarity positive at points 49 with respect top'o'ints 50. Points lfl are" connected through line 31, catnode-43-a nd igniting electrode 30-; "to the-cathode 28 of thyratron I8, and points 50 are connected through the capacitance's '35 -'and3'4"and a 'line 33-to the grid 32 of thyra-tron r8. 4 Therefore, with nocharg'e on condenser "3401 condenser 35, the grid 32*will normally be negative with respectto the cathode 28"durin'g the half cycles of? the alternating potential from lines 14 and 15 when the anode 28 is positive with: respect to "the camode l8 0f "the thyratron F8. Due te -"the" nullifying transformer 48 and connection of the grid 38 to the point 4| of line 33, the grid 38 of thyratron 19 will also normally be negative with respect to its cathode 29 during the half cycles when the anode 24 is positive with respect to the cathode 29. Resistances 51, 52, 53 and 54 are arranged in series across the points 49 and 58 of the constant potential source 38, and the resistance 55 is inserted in the line, 58 running through the capacitors 35 and 34 to the control grids 32 and v 38 of thyratrons l8 and [9, respectively. Re-

mary connected across the power source lines sistance 53 is of the potentiometer type. By adjustingan :arm 51 of the potentiometer resistance 53, fine adjustments may be made in the amount of negative grid bias at the grids 32 and 38. Resistance 52 'isof the variable type and has an adjusting arm 58 connected to one side of a switch 59; "The other side of switch 59 is connected to a point 68 in line 56 between resistance 55 and capacitance 35. When the switch 59 is closed, it shunts out resistances 55 and 5| in the grid circuit of thyratrons 18 and I9, therebysubstantially reducing the negative bias on grids 32 and 38. The amount of this grid bias reduction may be varied by adjusting the arm 58 along the remains open.

across the condenser 34 at points '62 and 83. In the oscillator 6|, a transformer 64, having its pri- I4 and I5, has the terminals of its secondary coil 86 connected to the anodesof a full-wave rectifying device 61. A smoothing condenser "88 is arranged across a cathode 89" of the rectifying device 61 at point 10, and the midpoint H of the secondary coil 68 by lines 12 and 13 connected at point 14. Voltage divider resistances 15 and 16 are connected in series across the points 10 and". The cathode 69 of the rectifying device,

*GI'is connected through point "It and potentiometer resistance 11 to the: point -82 in the line 33 on one side of the condenser 34; Point 63 on theother side of the condenser 341s connected 4 to point 18 between voltage divider resistances "I5 and 18.

connected across a condenser 34 with its anode A gas-filled discharge device 19 is connected through an inductance coil 8| to point 62 and its cathode 82 connectedto the point 63. Due to the directional current characteristics ofthe rectifying device 61, the condenser 34 will be made to charge with the side 62 positive and the sidefiilnegative. The rate of charge on condenser may be adjusted by the potentiometer resistance 11.

84' and 85 of a' balanced bridge rectifying device 86 to the point -14. Becauseof the voltage divider resistance 16, the'grid '83willnormally be negative with respect to the cathode 82 ot the gase= ous'discharge device 19;thereby preventing the discharge-device I9 from-firing. 'Points8l and of the balancedrectifyingbridge'BG are connected to the ends of asecondary-winding 890i an impulsetransformer 90-whose 'primarywindlng 9| connected in series with'a secondary coil 92 of a transformer 93 whose primary coil 94 is connected across the power source lines 14 and I5:

A potentiometer resistance 95 in parallel with a condenser 961s connected in series with another potentiometer resistance 91in the series circuit The switch 59 normally A grid 83 of the gaseous discharge device 191s connected to points secondary B9 of the transformer 90 may be varied. The rectifying devices of the balanced rectifying bridge 83 are positioned to make a positive pulse appear at the grid 83 of the gaseous discharge device 73 once for each half cycle of the alternating potential from lines I4 and I5. Because of the rate of charge on condenser 34 as explained above, the voltage versus time picture across the condenser 33 may be shown by curve 98 in Fig. 2. The point 99 on curve 08 designates an amount of charge on the condenser 34 effected during a halfcycle time interval of the alternating potential source from lines I4 and I5. The potentiometer resistance 91 is adjusted to cause the positive pulse from the impulse transformer 30 to appear at the grid 83 at the end of each half-cycle of the alternating potential source of lines I4 and I5, thereby triggering the gas discharge device I9 causing it to conduct current so as to discharge, nearly instantaneously, the condenser 34, the charge of which has just reached, point 99 (Fig. 2). Due to the inductance coil 8I, the drop in potential of condenser 33 will, experience a slight reversal to the point I to insure extinguishing the gaseous discharge device I9. When the gaseous discharge tube I9 is extinguished, the condenser 34 will again charge in a similar manner, as explained above, and shown by line IOI (Fig. 2) for the next half-cycle of the alternating potential from lines I4 and I5, until it reaches a point IOI' at the end of the next half-cycle, at which point the impulse transformer again triggers gaseous discharge device I9. This periodic charge and discharge of condenser 34 will continue in a cyclic manner twice for each flow cycle of the alternating potential source and is shown in Fig. 3 where curveI02 represents the potential cycles from alternating lines I and I5, and the saw-tooth potential curve I03 represents the corresponding potential verus time picture appearing across the condenser 34.

A step potential generator :34 is arranged to charge the condenser 35 through lines I05 and I05 connected at points l9! and IE3 across the condenser 35. A primary coil I09 of transformer I I0 connected across the alternating power source of lines M and I5 has its secondary coil III in series with a half-wave'rectifying device I I2, a potentiometer resistance H3, a current limiting resistance i M and a step control capacitance I I5. Because of the half-wave rectifying device II2, the condenser H5 will receive a charge only during alternate half-cycles of the potential source from lines I t and I 5 so that'polarity across the condenser I I5 will always be in the same direction. The negative side of condenser II5 is connected at point IE5 through line I05 to point I08 on the negative side of condenser 35, and the positive side of condenser II5 is connected to an anod I ll of a gaseous discharge device II8 having a cathode H9 connected by the line I35 to point I3? on the negative side of the condenser 35. A transformer I having a primary I2I connected across the power source lines I 4 and I5 has a secondary I22 in series with a'half-wave rectifying device I23. A resistance I23 and smoothing capacitance are are connected across th secondary winding I22 and rectifying device I23 at points I25 and I26. Due to the position of rectifying device I23, the point I25 will normally be positive with respect to the point I26. The positive point I25 is connected to the cathode H9 of the gaseous discharge device H8 at point I 21. The negative point I 26 is connected through thesecondary winding, I28 of impulse trans-1 6 former I29 to a grid I30 of the gaseous discharge device H8. The grid I30 will, therefore, normally be more negative than the cathode .I I3, thereby preventing th gaseous discharge device I I8 from firing. A primary I 3| of the impulse transformer I29 has one of its ends connected to a point I32 between the secondary 92 of transformer 93 and the primary iii of the impulse transformer 90. The other end or theprimary I3I of impulse transformer I29 is connected to a point I33 between the potentiometer resistances 95 and 91. Because of transformer 93, the impulse transformer I29 will eiiect one positive pulse and one negative pulse at the secondary I28 for each complete cycle of the alternating potential from lines M and 5. The pulse which makes the grid I30 more negative with respect to the cathode H9 will hav no effect on the discharge device IIO, but the pulse which is positive with respect to grid 33 and cathode II 9 will trigger the discharge device I it causing it to discharge the condenser I l5 into the condenser 35. When the charges between condensers I25 and have equalized, the potential between cathode H9 and anode ill will be zero, thus extinguishing the discharge device H8. The condenser II5 will then continue to charge to a higher potential. By adjusting the potentiometer resistance 95, the point in the cycle of the alternating currentpotential from lines it and I5, at which the positive pulse will occur in the secondary I28 of impulse transformer we, may be varied. In this instance, potentiometer resistance 95 is adjusted to cause discharge device H8 to be triggered at the end of each positive half-cycle of the alternating potential source from lines It and I5. Referring to Fig. 4, the potential versus time curve onthe condenser H5 will appear as line I30 due to the adjustment of potentiometer resistance H3. If, for example, the condenser H5 is a /2 microfarad condenser and the condenser 35 is a two microfarad condenser, and the maximum potential across transformer secondary III is 500 volts and varies as line I35, the charge on the condenser H5 in the positive half-cycle of the alternating potential from lines I 4 and I5 may be sufficient to create a potential of 50.volts across condenser '5 shown by point I36 on curve I34 (Fig. 4), at which point the discharge device H8. is triggered to discharge the condenser II5 into the condenser I35. Because of the difference in capacity between condenser 35 and H5, a voltage of 10 volts will be created across the capacitor 35 in a nearly instantaneous operation. The voltage across the condenser H5 will also be 10 volts as shown by the point I36 (Fig. 4). In the next positive half-cycle, the condenser II5 will again be charged by approximately volts since it is still operating at a relatively small potential compared to the transformer potential and will appear as a total of volts, as shown at point I37, at the end of the positive cycle when the gaseous discharge tube I I0 is again triggered to discharge the condenser II5 into the condenser 35, raising the voltage across the condenser 35 to approximately 20 volts, which will also be the voltage across the condenser H5 and may be shown by th point I31. The condenser II5 will continue to charge and discharge in a cyclic manner, building up the potential across the condenser 35 in approximately equal increments, each increment having a time-duration of-two half-cycles of the by the curve I39'in'Fig. 3.

alternating potential source,- and may be shown The condenser. 3551s L periodocally- -dischargedby a step-timing counter I 40,. The isteprtim-ing ooiuiteni 4.0 has a gaseous discharge device MI connected aorossthe condenser 35. The anode I12 of thegaseous discharge-device 114i is. con-I nected to :a point lotion thewpositive side of the condenser 35, and the ,cathode I44 oi the gaseou discharge device I4I isconnected .at point I415 to the-negative side of the condenser '35. A constantibiasing potentialsource'is connected across grid time and cathode I144 101' the gaseous 1.diS-' charge device I. In this instance, the constant potential source comprises :the transformer IMhaving a primary I. connected across the alternating potential source :trom lines It and I and a secondary coil 148 in series with a rectifying :device I49-with ,a smoothing condenser I50 and resistance MI. The condenser I50 and resistance I5.I are connected in parallel across the series-connected rectifying device I49 and secondary coil I48. The positive side I52 of the constant potential source is connected through line I 53 to the cathode I44 01 the gaseousdischarge device MI. The negativeside I54 is connected through a secondary coil I55 of a triggering transformer I56 to the grid I45 of the gaseous discharge device I4I. Since the grid I45 will normally be maintained negative with respect to the cathode I44, the gaseous discharge device I4I will normally not conduct, and will, therefore, permit the step potential onthe condenser 35 to progressively increase, as explained above, until a triggering pulse from the transformer -I56 makes the grid I45 sufiiciently posithe with respect to the cathode I44 to cause the gaseous discharge device I4I to conduct and thereby discharge thecondenser 35. To obtain the triggering pulse, transformer 156 has the ends of its primary "coil I51 connected between ananodel58 0t agaseous discharge device 159 and the positive side 3160 of ajtiming condenser I6I whose negative side I62 is connected to a cathode I63 of the gaseous discharge device 159. Across 'theterminals 160 and IE2 are connected in series a half-waveirectifying device I64 potentiometer resistance I65, and the secondary r coil I66 of a transformer I61 whose primary I68 is connected across the alternating power source lines Id andifi. rectional characteristic of the half-wave rectifying device I6,4,,current will flow to increase 3 the potential across the condenser I6I during a1- ternate half-cycles of the alternating potential source of lines I4 and'I5, thus making theanode I58 progressively ,more positive with respect to the cathode I63 or the gaseous device I59). The

wave rectifying device I12 .in series rwithiaisecondary I13 oi transformer I14, whose primary I15 .is connected =across-the alternating power source lines I4 and 1l5,a smoothing capacitance I I16 anda resistance I11 in p r llel acrossxth series rectifying device 1- 12 and n former: c

ondary 1.13. The rectifyi g device A12 i e ranged so that theqmtentiali of thegrid :I-GBnviil normally be highly negative with nespectztorthe cathode mic! in Because of the unidi 1 10 0! the impulse transformer .I'1 I-h one side connected-to thepoint I33 between tentiometer resistance 61 "and potentiometer resistance 65. The other side of *the primaryill is connected to a point I32 between the secm r 11 will prod ce twopuis s .fo e h complete cycle of the alternating potential ,fromlines I4 and I5 one positive and o e negative. The negative pulse from thesecondary I11! make he g id N9 nc a me ynegative with resp ct to 'thev atho 163 sot at he gaseou dischar vi I59 wi l not conductiirh p s tive nuisahi e his nqa di ection tending to overcome the negative biason the grid 169; has a masmVi d which is nsumcient I of itself to tr g er the easeusdevice I 59, but the char e on the condenser 1 .lsinser es opp sition to the votentialacros sistence m. Th s. asthe pote tialoacross econ enser lei'continuest increase. the new tive bias of grid I69 due. to thezpotential across the resistance .llicontinues ,to decreaseat a rate etermined by t ettine on th p tentiometer resistanc r155, When the negative mason slid 1 I69 vlacon cies i ufficiently s all the positive "pul om the second ry I 11 at mpulse transformer I IH is 'vsufficient to suddenly m k the gri sutficientlif .positive with respect "to the ,cathode I63 tocause hflgaseous dis harge de ice list ner:- uc to di cha ge the conden er 45 I. The com enser A 61 will ischarge rapidly throuehithe primary .151 of 2th transtormer I56 thereby omings trig e in pulse in secondar I of such a iiwlarity and magnitude as to make the 1 rid I45 sufficiently positive with respect to. the cathode 444 to cause the gaseous rdischargerdevice .-I 41 to conduct, thereby discharging the-com denser 35. Since the impulse transformed] H is arranged to produce impulses ,at the 15 m? "time as theirnpulse transformer I29, atriggering-impulse-willioccur at theiend o-f-the cycleof alternating potential from the lines I4 and I5. The

discharge of the condenser 35- will, therefore,

occur at a point at the end of'a complete cycle,

and :make the discharge appear graphically as at mt oflthestep-timing curve I39 in Fig. 3. The number 101 cycles between discharges .of the condenser may-ioe varied :inrthis instance .by ad? l i g m antiometer resistance 1 I65.

To control maximum possible potential across the condenser 35, a gaseous discharge d vic nlil of he vo t e regulator typ is com heated across condenser 35 at points 1I43and ,Referring now to the general operation of the invention, it may begraphicall-y shown as at I82 of Fig. 6. The alternating potential appearing across the anode 2'3 and cathode 413 of ,ig-nitron I 6-may be ,shown by curve :I 63, which-11s also ,the-

potential ;:picture appearing across the anode 2'6 and the cathod 26 of the thyratron I6. The curve I 84 shows; generally the potential :at which theigrid'rfl will cause the thyratron to conduct. When the switch 58 of the constant potential source 16 is open, the resistances 55,15Ivand part of the resistance 82 cause the grid 32 -01 "thyratron -I'8 .to (have a :large negative bias 165 (Fig.

3) with riespeetitotheicathode 26. Insuch case, than: orithepotential from 1m 3M and the maximum potential from the condenser 35 are insuiiicient at any time to reduce the negative bias of grid 32 to the point where the thyratron I8 will conduct. However, when the switch 58 is closed, thereby shunting out the resistances 55 and and 'a part of 52, a. base biasing potential I86 is effected at grid 32 of thyratron I8. in such case, the sawtooth potential I 81 across the condenser 34 will cause the potential of the grid 32 to become sufficiently positive at the point I88 to cause the thyratron I8 to conduct, and thereby causing the ignitron I6 to fire at a correspondingpoint so as to pass a current pulse I89 by means of transformer I0 through the load. As the step potential I99 across the step condenser 35 builds up, the bias of grid 32 will become sufliciently positive to cause the thyratron I8 to conduct at progressively advancing points in the positive half-cycle of the alternating potential I83 as shown by I 9| I92 and I93, and will cause corresponding current pulses of progressively increasing intensity and duration as shown at I95, I96 and I91, respectively, until the condenser 35 is discharged to repeat the cycle. Due to the inverse parallel connection of ignitron I1 and the nullifying transformer 40, a similar picture will appear for each negative half-cycle of the potential source 83 so as to produce negative pulses of current I99 through the load. The resulting heat picture of the weld materials at the load will, therefore, appear approximately as curve 200 (Fig. 3). By adjusting the variable resistance arm 60 of the constant potential source 36, the initial point I88 (Fig. 3) in each cycle at which the thyratron It will conduct so as to cause ignitron I6 to conduct may be varied, thereby varying the intensity of the initial current pulse I89 and thus the minimum temperature 2M in the heat cycle 206 at the Weld load. By adjusting the potentiometer resistance I65 of the step timer I40, the length of the heat cycle may be varied. By adjusting the potentiometer resistance H3 of the potential step control I04, the slope 203 of the heat curve 290 may be varied. A heat cycle adjustment, therefore, most suitable to any particular material may be obtained.

This invention is not limited to the particular details of construction and processes described as many equivalents will suggest themselves to those skilled in the art. It is accordingly desired that the pending claims be given a broad interpretation commensurate with the scope of the invention within the art.

What is claimed is:

l. The system comprising a transformer having a secondary winding for supplying power to a load and a primary winding, means for connecting said primary winding to a source of a1- ternating current including two gaseous discharge devices in inverse parallel connection each having an anode and a cathode and a control electrode, means for energizing said control electrodes to cause said gaseous devices to discharge thereby connecting said alternating current source to said primary winding, saw-tooth potential Wave generating means, step potential generating means connected to said saw-tooth potential generating means in a manner to provide a progressively increasing potential base for said saw-tooth potential generating means, means connecting said last-two-mentioned means to said energizing means so as to cause said energizing means to energize said control electrodes at progressively advancing points in the 10 cyclic pattern of said alternating current source, each of said advances corresponding to a potential step of said step generating means, and time control means for returning the potential of said step generating means to a base potential position.

2. The system comprising a transformer having a secondary winding for supplying power to a load and a primary winding, means for connecting said primary winding to a source of alternating current including two gaseous discharge devices in inverse parallel connection each having an anode and a cathode and a control electrode, means for energizing said control electrodes to cause said gaseous devices to discharge thereby connecting said alternating current source to said primary winding, saw-tooth potential wave generating means, step potential generating means connected to said saw-tooth potential generating means in a manner to provide a progressively increasing potential base for said saw-tooth potential generating means, adjustable potential means connected to said last.- two-mentioned potential generating means to provide a base potential for said last-two-mentioned potential generating means, means connecting said last-three-mentioned means to said energizing means so as to cause said energizing means to energize said control electrodes at progressively advancing points with respect to a base energizing point in the cyclic pattern of said alternating current source, each of said advances corresponding to a potential step of said step generating means, and time control means for returning the potential of said step generating means to said base potential position.

3. The system comprising a transformer having a secondary winding for supplying power to a load and a primary winding; means for connecting said primary Winding to a source of a1- ternating current including two ignitrons in inverse parallel connection each having an anode and a cathode and a control electrode; means for energizing said control electrodes including a thyratron for each of said ignitrons, each of said thyratrons having an anode and cathode and control grid; time control means for causing said thyratrons to energize said control electrodes at progressively advancing points in the cyclic pattern of said alternating current source, said time control means including two capacitances and an adjustable potential source connected in series across said control grid and cathode of each of said thyratrons, means for cyclicly charging and discharging one of said condensers in a sawtooth potential time pattern, means for charging the other of said condensers in a stepped potential time pattern; and time control means for periodically discharging said last-mentioned condenser thereby providing at said lastmentioned condenser a cyclic stepped potential time pattern.

4. The system comprising a transformer having a secondary winding for supplying power to a load and a primary winding, means for connecting said primary winding to a source of alternating current including two ignitrons in inverse parallel connection each having an anode and a cathode and a control electrode; means for energizing said control electrodes including a thyratron for each of said ignitrons, each of said thyratrons having an anode and cathode and control grid; time control means for causing said thyratrons to energize said control electrodes at progressively advancing points in the cyclic pattern of said alternating current source,

"said time control mean including two capacitances and an adjustable potential source connected in series acrosssaid control gridand'c'at'hode of each of said thyratrons, means an char ing one of said condensers'in a direction making said control grid more positive, means for'c'ontrolling the rate of charge of saidcharging means, control means for discharging said condenser once for each half-cycle of said alternating'current source, means for charging the other of said condensers in a direction making said' control grid more positive, means'for causing'said'oharging means to charge saidoth'er condenser in'steps, each of said steps having a duration or an even number of successive half-cycles of said alternating current source; and time control" means for periodically discharging said last-mentioned condenser thereby providing at said last-mentioned condenser a cyclic stepped potentialtime pattern.

5. In a step timing welding control systemlor use with an alternating potential source, a gaseous discharge device having an anode; a cathode and a control grid; time control-means connected to said control grid and cathode to cause said parts to .be welded together, producing, an alternating electric current in the: region or the junction oiitne parts; and varying. the intensity or saidhcurrent for a. plurality of. cycles; each. cycle varying'inintensity from a minimum value gradually increasing with time to a maximum value which exists withinonly one period or said alternating current and then returning within one period ofsai'd alternating'current to said minimum value;

7. In a welding system, meansfor urging the parts to be welded together; means for producing' an alternating electric current in the region or the junction" of the parts; and" means for varying the intensity or said currentr'or a plurality of cycles; eachu'cycle varying in intensity from a minimum-value gradually increasing, with time to a maximumyamewhicn exists within only one periodi'of said alternating current and then returning substantially within one period or said alternating current tosaid. minimum value.-

DONALD P. FAULK.

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